This effect is known as Van Der Waal forces. This youtube video shows a short visualisation of the effect, allowing you to get an idea of what is actually happening and visualise it.
Conclusion: For groups one and two, the melting points increase as you go up the group due to the metallic bond strengthening. For the non-metals and transition metals the melting points increase as you go down the group. As you go across a period, the melting point increases until you hit group 6, before it begins to decrease again.
The sites I used for references are linked throughout the page. Click the little green arrows for links to the page and more info.
This effect is known as Van Der Waal forces. This youtube video shows a short visualisation
of the effect, allowing you to get an idea of what is actually happening and visualise it.
For groups one and two, the melting points increase as you go up the group due to the metallic bond
strengthening. For the non-metals and transition metals the melting points increase as you go down
the group. As you go across a period, the melting point increases until you hit group 6, before it begins
to decrease again.